Colout TV Fundamentals

 Electrical output of camera tube is not linear function of
light input.
 Light output of picture tube is also not a linear function of
electrical input.
 Exponent of transfer function called Gamma.
Gamma Correction

 γ = 1
 System is linear.
 Overall γ = 1.2 gives slightly more
pleasing picture.
 B/W PT - γ = 2
 Colour PT - γ = 2.2
 Hence gamma of transmission
chain should be 0.5.
Gamma correction

 Delta Gun Colour Picture Tube
 Gun-in-Line or Precision-in-line Colour Picture Tube
 Single gun or Trintron Colour Picture Tube
Colour Picture Tube

 Radio Corporation of America RCA
 3 separate guns for 3 colours.
 Guns equally spaced at 120 ,tilted towards axis.
 Screen coated with 3 different phosphors for R, G, B.
 Triad contains 1 dot of each colour.
 3,33,000 triads.
 Eye integrates 3 colour information to give a sensation of
combined hue.
 Shadow mask.
 One hole per triad.
 All stray electrons (80%) get collected by mask.
 Hence need higher EHT.
Delta Gun Colour Picture Tube

Gun viewed from base
120º120º
120º
Electron
Gun
Tube
Neck

Electron Guns with mask and screen

 Convergence is difficult.
 Very elaborate arrangements to overcome it.
 Focus not sharp over entire screen.
 As focus and convergence planes for three guns
separated by 120º are not same.
 Mask passes only 20% of electrons.
Disadvantages

Precision-in-line Colour Picture Tube

 3 Guns in line simplifies convergence adjustment.
 Colour phosphors in form of vertical stripes.
 Distance same as in Delta gun.
 More efficient.
 Larger % of electrons pass through mask.
 Fewer convergence adjustments.
 Most widely used.
Precision-in-line Colour Picture Tube

 By Sony Corporation, Japan
 Single gun having three cathodes.
 Simplified construction as only one electron assembly.
 Three phosphor triads arranged in vertical strips.
 Each strip is few thousandth of a cm.
 A metal aperture grill has one slot each triad.
 Grill has greater electron transparency.
 Three beams appear to emerge from same point.
TRINTRON Colour Picture Tube

 Each beam must fall at center of corresponding dot.
 Irrespective of position of triad on screen.
 Circular magnets called purity magnets on neck of yoke
does the alignment.
 If all tabs moved together, they change direction of field.
 If tabs are separated, magnetic field reduces.
Purity and Convergence

 2, 4, 6 pole magnets to achieve collective or individual beam
deflection.
 Error may also occur if yoke is not positioned properly.
Purity

 Technique that all electrons hit the same part of the screen
to produce 3 coincidental rasters.
 Errors–
 Non-coincidental convergence plane.
 Non-uniformity of deflection field.
 Flat surface of picture tube screen.
Convergence

 Correction:
 Static and dynamic convergence.
 Static convergence by permanent magnet.
 Once correctly set, brings the beam into convergence in the
central area of the screen
 Dynamic convergence converges beam over rest of the
screen.
 Dynamic convergence achieved by additional winding in
series with yoke coil.
 Achieved by continuously varying magnetic field.
 Instantaneous strength depends upon position of the spot
on the screen.
Convergence

• Static pincushion may cause purity problem.
• Dynamic pincushion correction.
• This stretches the horizontal width and vertical size of
raster at edges.
Pincushion correction

• Due to Earth magnetic field and other nearby magnetic
fields, mask and mounting frame may get magnetized.
• Will cause purity error causing in colour patches.
• Can be repaired by influencing the screen by an alternating
magnetic field which gradually reduces to zero.
Degaussing

• At switch ON, thermistor is cold and has high resistance.
• More AC passes through coil and varistor, causing alternating
magnetic field.
• Thermistor gradually heats up, reducing its resistance.
• More current flows through thermistor.
 Lesser current through coil and varistor.
 Heats thermistor further
• This continues till
 current through thermistor becomes maximum.
 Current through varistor and coil gradually reduces to zero.
• Components chosen to give initial surge of 4ampere to D.Coil.
• Final current less than 25mA in less than a second.
Automatic degaussing

• R G B must combine to give to give white.
• 3 phosphors have different efficiencies.
• 3 guns may not have identical emission and cutoff point
(Ip/Vgk).
• Tint appears instead of pure grey shades for monochrome
transmission.
• Suitable adjustment required to produce correct monochrome
information with no colour tint for all settings of contrast
control.
Grey Scale Tracking

• Appearance of tint instead of pure grey shades in areas of
low brightness.
• Necessary to bring cutoff points in coincidence.
• Achieved by making screen grid voltage (1st anode) different
for each cathode.
• Potentiometers are normally provided in DC voltage supply
to 3 screen grids.
Grey Scale Tracking
Adjustment on low light

• All levels of light must be correctly reproduced.
• Need to Compensate for slightly different slopes and also for
substantially different phosphor efficiencies.
• Achieved by the video signal Y drive to the 3 guns.
• Red has lower efficiency, maximum video signal Y fed to red
cathode.
• By potentiometer B and G inputs are varied to produce
optimum reproduction of high lights.
Grey Scale Tracking
Adjustment on high light

 3 Systems---
 American NTSC(National Television System Committee)
 German PAL( Phase Alteration by Line)
 French SECAM(Sequential couleures a memoire)
 All systems are good.
 India adopted PAL for 625 line CCIR-B standard for B/W.
 NTSC and PAL being similar are dealt together.
Colour signal
transmission and reception

 Should accommodate hue and saturation in same band of
7MHz.
 Colour signal should not disturb B/W information and vice versa.
 Done by Frequency interleaving.
Colour signal transmission

 Video signal band bears a definite relation with scanning
frequencies.
 Energy content of video signal is contained in individual energy
“bundles”.
 Bundles occur at harmonics of line frequency.15625Hz
 Components of each bundle separated by multiple of field
frequency. 50, 100, 150…
 Each bundle has peak at exact line harmonic gradually reducing
in amplitude on either side.
 Amplitudes of bundles reduce towards higher harmonics of line
frequency.
Frequency interleaving

Energy bundle
Amplitude
fH = 15625Hz

 Vertical sidebands contain lesser energy than Horizontal
because of lower rate of scanning frequencies.
 Overall Energy contents decreases to very small value beyond
3.5MHz from picture carrier.
 Part of B/W BW unused at spacing between the bundles.
 Used for colour energy bundles.
Energy bundle

 Colour subcarrier so chosen that side band energy
frequencies exactly fall between harmonics of line
frequencies.
 Hence colour subcarrier chosen to be odd multiple of half
of line frequency.
 To avoid any cross talk between B/W and colour info,
higher portion of B/W BW is chosen to place coloue BW.
 Hence Colour subcarrier is 567 times half line frequency.
 PAL = 567x 15625/2 = 4.43MHz
 NTSC = 455x 15750/2 = 3.58MHz
Colour Energy bundle

Interleaving of Energy bundles

 Study show that eye can perceive colour in object areas of 1/25th
of screen width or more.
 For smaller areas, Eye can only perceive its brightness.
 Between 0 – 0.5MHz, all colours can be seen.
 0.5 – 1.5MHz, only two primary colours seen. G, B.
 Eye can not distinguish purple and green-yellow hue.
 For smaller objects, colour not visible, only shades of grey.
 Hence colour B/W = ±1.5MHz = 3MHz (DSB-SC)
Band width of Colour Signal

 (B-Y), (R-Y).
 Carrier = 4.43MHz.
 Second carrier by giving 90º phase shift to carrier.
 Called Quadrature modulation.
 Resultant subcarrier phaser called chrominance signal C.
 Instantaneous value of C represents colour saturation at that
time.
 Phaser of C varies from 0º to 360º , represents hue.
Modulation of Colour Signal

 (R-Y) = R - 0.59G – 0.3R – 0.11B
= 0.7R – 0.59G – 0.11B
 (B-Y) = B - 0.59G – 0.3R – 0.11B = 0.89B – 0.59G – 0.3R
Modulation of COLOUR Signal

 Pure red –
 R = 1v
 B = G = 0v
 (R-Y) = ?
 (B-Y) = ?
(B-Y)t
-(R-Y)t
(R-Y)t
-(B-Y)t

 Pure red –
 R = 1v
 B = G = 0v
 (R-Y) = 0.7R
 (B-Y) = -0.3R
0.7
-0.3
0.76
(B-Y)t
-(R-Y)t
(R-Y)t
-(B-Y)t

 Pure red –
 R = 1v
 B = G = 0v
 (R-Y) = 0.7R
 (B-Y) = -0.3R
 C = √[(R-Y)2 + (B-Y)2]
 θ = tan-1 [(R-Y)/(B-Y)]
C(t)
θ(t)

 Similarly can be found for pure Blue, Green, Magenta, Cyan,
Yellow

 (R-Y) and (B-Y) modulate colour subcarrier using DSB-SC.
 Avoids interference due to colour subcarrier.
 Avoids power wastage.
 In B/W transmission, (R-Y), (B-Y) are zero. No colour carrier.
Hence no chrominance signal.
 But we need to generate Colour subcarrier at receiver for
detection of DSB-SC.
 It should have same frequency and phase.
 Hence 8-11 cycles of subcarrier sent as pilot for synchronization.
 Rides the back porch of Blanking pulse.
Colour burst signal

 Does not interfere with sync
as small in amplitude and
occurs after the sync.
 Gated out and used to sync
colour subcarrier.
 H and V sync also derived
from it as they bears a
constant relation with it.
Colour burst signal

 (R-Y) and (B-Y) modulate 4.43MHz colour subcarrier and
4.43MHZ with 90º phase shift.
 Gives C, θ(t)
 C when added to Y, rides over Y.
Chrominance Signal Weighing Factor

 Over-modulation must be avoided.
 Over modulation reduced by reducing (R-Y) and B-Y) before
modulating colour carrier.
 (R-Y) and B-Y) scaled down.
 (R-Y)’ = 0.877(R-Y)
 (B-Y)’ = 0.493(B-Y)
 And then modulate the carrier.
Weighing Factor

 New values still over modulates.
 In real life, such pure colours never occurs.
 Hence amplitude levels never reach shown peak values.
 No over modulation in real signals.
 At receiver, (R-Y) and (B-Y) need to be scaled up to bring
them to original value.
 Needs controlled amplification.
 Due to weighing, there is change in amplitude and phase of
various colours.
Weighing Factor

 Colour B/W reduced to 2MHz.
 Eye’s resolution of colours along reddish blue –
yellowish green axis on colour circle is much less than
colours which lie around yellowish red – greenish blue
axis.
 Two new signals Q and I chosen to represent these areas.
NTSC SYSTEM

 33º counterclockwise to (R-Y).
 Maximum colour resolution.
 (R-Y) = 0.74.
 (B-Y) = -0.27
 I = 0.74(R-Y) – 0.27(B-Y)
 I = 0.60R – 0.28G – 0.32B
 Covers orange hue and blue-green (Cyan) hue.
 Eye is more sensitive to colours around it.
 More bandwidth is allowed for I.
I Signal

 33º counterclockwise to (B-Y).
 (R-Y) = 0.48.
 (B-Y) = 0.41
 Q = ?

Q Signal

 33º counterclockwise to (B-Y).
 (R-Y) = 0.48.
 (B-Y) = 0.41
 Q = 0.21R - 0.52G + 0.31B
 Covers reddish blue (Magenta) and yellow-green hue.
 Eye is less sensitive to colours around it.
 Less bandwidth is allowed for Q.
Q Signal

 0 to 0.5MHz – Both I and Q are active.
 0.5 to 1.5MHz – Q drops out. Only I remains.
 DSC-SC allowed for Q.
 BW = 1MHZ
 VSC allowed for I.
 Upper side band = 0.5MHz
 Lower side band = 1.5MHz
 BW = 2MHZ
 Total BW = 2MHz for colour, 6MHz for NTSC system.
Bandwidth of I and Q Signal

 500ns delay to y signal.
 Chrominance signal during active trace time.
 Burst signal during retrace time.
 Colour killer ckt kills video amplifier during retrace to
prevent burst affecting the picture.
 Done by application of blanking pulses to turn the bias of
amplifier off.
 Final colour amplifiers to compensate for weighing factor.
 R-Y boosted by 1.4
 G-Y boosted by 2.03
 B-Y attenuated by 0.7
NTSC Receiver

 Sensitive to transmission path difference which
introduces phase errors that result in colour changes in
picture.
 Phase changes can also take place when changeover
between local and TV network system take place.
 Chroma phase angle is also affected by the level of the
signal while passing through various circuits.
 Cross talk between demodulator outputs at the
receiver can cause distortions.
 All above require automatic tint control with provision
of manual control.
Limitations of NTSC Receiver

 At Telefunken laboratory in the Federal Republic of
Germany.
 Phase error susceptibility of NTSC system has been largely
eliminated.
PAL Colour system

 Weighted (R-Y) and (B-Y) are used.
 (R-Y) and (B-Y) both are given same B/W of 1.3 MHz.
 Gives better colour reproduction.
 Vestigial Sideband modulation used.
 USB – attenuation slope starts at 0.57MHz.
 (5-4.43 = 0.57MHz)
 LSB – Extends to 1.3MHz before attenuation begins.
 Colour subcarrier = 4.43361875MHz.
 Gives better cancellation of dot pattern interference.
Features of PAL Colour system

 (R-Y) and (B-Y) modulated with subcarrier using QAM as in
NTSC but with a difference.
 Phase of one carrier is 0º.
 Phase of other carrier shifts between +90 º to -90 º at alternate
lines .
 Hence the name Phase Alteration by Line(PAL).
 This cancels hue errors resulting from unequal phase shifts in
the transmitted signal.
 Shifting of phase occurs during line blanking interval to avoid
visible disturbance.

 Weighed (B-Y) and (R-Y) are-
 U = 0.493(B-Y)
 V= 0.877(R-Y)
 CPAL = USinωst ± Vcosωst
 = √(U2 + V2) Sin(ωst ± θ)
 Where θ = V/U
+

 PAL receiver requires three types of carrier.
 Subcarrier at 0º phase to detect U signal
 Subcarrier at +90º phase at every second line for V.
 Subcarrier at -90º phase at every second line for V.
 V demodulator must be switched at half horizontal line
frequency rate.
 Hence PAL Colour burst has two components.
1. -(B-Y) (same as NTSC), but of amplitude 1/√2 of NTSC burst.
2. (R-Y) component which is reversed in phase from line to line,
amplitude same as –(B-Y)
PAL Burst

 Resultant burst swings ± 45º about –(B-Y) axis from line to line.
 Sign of (R-Y) burst always same as that of (R-Y).
 Hence called swinging burst.
PAL Burst

 Multiple path transmission results in phase errors.
 Called differential phase error.
 Changes hue in reproduced picture.
 PAL has built-in protection against phase errors.
 Provided picture content almost remains same from line to line.
Cancellation of phase errors

 Line N and N+1 response shown.
 U and V are detected on two separate detectors.
 Resultant R has same amplitude on both line.
No Error
Line N
NTSC Line
Line N+1
PAL Line

 Suppose R suffers phase error of angle δ .
 Resultant phasor will swing between R1 and R2.
 Phase error cancels if two lines are displayed simultaneously.
With phase Error

 Practically, two lines are displayed in sequence.
 Colours reproduced by two successive lines will be slightly on
either side of actual hue.
 Since lines are scanned at very fast rate, eye perceives a colour
that lie between two colours reproduced by R1 and R2.
 Hence colour viewed will be more or less the desired hue.
 PAL superior to NTSC in this front.
Cancellation of phase errors

 Small errors not visible to eye.
 Beyond a limit, eye starts to see the error in alternate lines.
 Remedy – Delay line employed to do the averaging of alternate
line colour information first then present the colour to viewer.
 Hence called Delay line PAL or PAL-D
PAL - D

 PAL-D removes phase errors and Differential phase error
problem .
 Manual hue control not needed..
 BUT..
 Delay line technique of reception results in reduction in
vertical resolution of chrominance signal.
 More complicated and expensive due to delay lines, adding,
subtracting…
 Receiver cost higher.
Merits and Demerits of PAL

 Earlier – 819 lines system with B/W of 14MHz.
 NOW –
 625 lines per frame.
 25 frames per second.
 50 fields per second
 Line frequency – 15625HZ
 Field frequency – 50Hz
 Video B/W – 6MHz
 Channel B/W – 8MHz
 Picture – FM
SECAM System

 Sound carrier 5.5MHz away from Picture carrier.
 Colour Subcarrier ---4.4375MHz away from Picture carrier.
 Y signal same as NTSC/PAL.
 Weighing factor –
 IDRI = -1.9(R-Y)
 IDBI = 1.5(B-Y)
SECAM System

 French “Sequential a Memoire”.
 Only one of two colour difference signal transmitted at a
time.
 (R-Y) transmitted in one line and (B-Y) in next line.
 Sequence repeated for full raster.
 Due to odd number of lines, (R-Y) and (B-Y) alternate each
picture.
 Subcarrier is frequency modulated by colour difference
signal before transmission.
 Magnitude of frequency deviation represents colour
saturation and rate of deviation its hue.
SECAM System

 Ultrasonic delay line of 64µs used as one line memory device.
 Produces decoded output of both colour difference signal
simultaneously.
 Modulated signals routed to their correct demodulators by an
electronic switch operating at line frequency.
 Switch driven by Bistable Multivibrator.
 Determination of proper sequence of colour lines in each field is
accomplished by Identification pulse .
 Ident generated and transmitted during vertical blanking
interval.
Principle of SECAM SYSTEM

 FM for colour.
 Phase distortions in transmission path will not change hue of
picture area as two carriers are at same phase..
 Limiter to remove amplitude variation.
 Subcarrier for (R-Y) = 282fH = 4.40625MHz
 Subcarrier for (B-Y) = 272fH = 4.250MHZ
 Suppresses dot pattern interference on B/W TV.
Modulation of Subcarrier

 Colour difference signals band limited to 1.5MHZ and pre-
emphasised.
 Linear deviation for (R-Y) = 280DR KHz
 MAX DEVIATION: 500KHZ and 350KHZ as below.
 +(R-Y) = -500KHz
 -(R-Y) = +350KHz
 Linear deviation for (B-Y) = 230DB KHz
 MAX DEVIATION: 500KHZ and 350KHZ as below.
 +(B-Y) = +500KHz
 -(B-Y) = -350KHz
Modulation of Subcarrier

 It is necessary for receiver to determine which line is being
transmitted.
 Ident pulses are generated during Vertical blanking pulses.
 Ident signal is carrier duly modulated by a saw-tooth signal.
 Sawtooth signal is positive going for DR and negative going
for DB.
 At receiver, generate positive and negative control signal
after detection.
 Regulates instant and sequence of switching.
Ident Signal Generator

 Y created as in PAL.
 DR and DB fed to switch for switching per line.
 Sync pulse generator generates sync.
 Added to Y.
 Controls signal and subcarrier switching through switching
control.
 Also controls Ident generator to be added to DR and DB during
blanking.
 DR and DB fed to FM modulator after 1.5MHZ LPF and pre-
emphasis.
 HF pre-emphasis increases amplitude of subcarrier as its
deviation increases, to further improve S/N.
SECAM CODER

SECAM DE-CODER
(R-Y)
(B-Y)
VR, VG, VB
(R-Y)
(B-Y)
VR, VG, VB
VR, VG, VB
(R-Y)
(B-Y)
VR, VG, VB
VR, VG, VB

 By Japan Broadcast Company NHK.
 Offers wide screen format.
 Does not show scan lines even from close.
 Has twice vertical and horizontal resolution.
 Improves colour rendition.
 Stereophonic sound.
 1125/60 HDTV comparable to 35mm theatre film.
 22GHz band for HDTV direct broadcast to TV.
HDTV

 Three parts:
 Generating equipment Camera, VCR
 Transmitters and links – microware, satellite.
 Large screen TV receiver, projection TV
 Highly sensitive equipments developed for each of above.
HDTV

 Aspect ratio 19:9.
 Gives channel B/W = 26.7MHz.
 60Hz field preferred to reduce flicker.
HDTV

Colout TV Fundamentals

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